Meniere's disease and other incapacitating vestibular disorders derive from an abnormal input from vestibular afferents. We have little understanding of how afferent input is integrated. The hypotheses developed in this proposal address directly morphophysiological correlates of vestibular afferents. One central hypothesis to be tested is that synapses from different classes of vestibular primary afferents are distributed differentially on vestibuloocular (VOR) and vestibulospinal (VST) neurons. Preliminary data suggests that there are markers that specifically label morphologically and/or physiologically distinct classes of afferents, e.g., calretinin. These experiments will extend this data, documenting significant correlations between the afferent's distal terminal morphology, physiological properties, and immunohistochemical staining characteristics. The first specific aim (SA) will test the hypothesis that calretinin stained calyxes are the physiologically distinct, irregular firing, low gain afferents. The first criterion is to demonstrate, by combining tracers with immunohistochemistry, that calretinin is restricted to calyx-only afferents. The second step is to demonstrate, by combining intracellular labeling and immunohistochemistry, that calretinin-containing primary afferents are irregular-firing, phasic, low gain afferents. SA2 will test the hypothesis that peripherin-stained boutons are the most regular firing, low gain afferents. We will utilize the steps and criteria outlined for SA1 to establish this morphophysiological and immunohistochemical correlation. SA3 will test the hypothesis that a subset of the population of afferent neurons with dimorphic endings stains with calbindin. The same morphophysiological and immunohistochemical criteria will be used. SA4 will use rigorous stereological measures to accurately access the number and proportion of afferents that stain with calretinin, peripherin, and calbindin. This will be the first unbiased determination of subpopulations of vestibular afferents. SA5 will test directly the hypothesis that VST neurons receive more of their afferent input from irregular afferents and that VOR neurons receive more of their afferent input from regular afferents. We will determine the ratio of close oppositions of immunohistochemical stained afferents on retrogradely-labeled VST neurons that project to the first cervical segment and on VOR neurons. The confirmation of the morphophysiological hypotheses will support quantitative models of afferent input and provide novel approaches to experimentally test new hypotheses about central integration. This could prove important for treatment of patients with vestibular disorders, such as those suffering with Meniere's disease or even normal aging.